The pressure to be uniform in attire (6)
I believe the answer is:
duress
'the pressure' is the definition.
(thesaurus)
'uniform in attire' is the wordplay.
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'attire' becomes 'dress' ('dress' can be a synonym of 'attire').
'u' going within 'dress' is 'DURESS'.
'to be' is the link.
(Other definitions for duress that I've seen before include "Constraint, force" , "Hardship" , "Compulsion by use of force" , "Use of force or threats, often vile" , "Excessive pressure" .)